UN agencies say 100,000 people are facing starvation in South Sudan and a further 1 million there are classified as being on the brink of famine. This is the most acute of the present food emergencies. It is also the most widespread nationally. Overall, says the UN, 4.9 million people - or 40% of South Sudan's population - are "in need of urgent food, agriculture and nutrition assistance".
"Many families have exhausted every means they have to survive," says the UN's Food and Agriculture Organization representative in South Sudan, Serge Tissot.
The basic cause of the famine is conflict. The country has now been at war since 2013 and more than 3 million people have been forced to flee their homes.
As World Food Programme country director Joyce Luma says: "This famine is man-made."
"The people are predominantly farmers and war has disrupted agriculture. They've lost their livestock, even their farming tools. For months there has been a total reliance on whatever plants they can find and fish they can catch," says Mr Tissot.
Crop production has been severely curtailed by the conflict, even in previously stable and fertile areas, as a long-running dispute among political leaders has escalated into a violent competition for power and resources among different ethnic groups.
As crop production has fallen and livestock have died, so inflation has soared (by up to 800% year-on-year, says the UN) causing massive price rises for basic foodstuffs.
This economic collapse would not have happened without war.
The UN considers famine a technical term, to be used sparingly. The formal famine declaration in South Sudan means people there have already started dying of hunger.
More specifically, famine can be declared only when certain measures of mortality, malnutrition and hunger are met.
The declaration of a famine carries no binding obligations on the UN or anyone else, but does bring global attention to the problem.
Previous famines include southern Somalia in 2011, southern Sudan in 2008, Gode in the Somali region of Ethiopia in 2000, North Korea (1996), Somalia (1991-1992) and Ethiopia in 1984-1985.
The possibility of three further famine declarations in Nigeria, Somali and Yemen would be an unprecedented situation in modern times.